Method for making a dispersion strengthened alloy article

ABSTRACT

A SUPERALLOY ARTICLE BASED ON AT LEAST ONE OF THE ELEMENTS FROM THE GROUP IRON, COBALT AND NICKEL IS PROVIDED WITH IMPROVED MECHANICAL STRENGTH PROPERTIES, PARTICULARLY AT ABOUT 1400*F. OR ABOVE, THROUGH AN IMPROVED POWDER METALLURGICAL METHOD. SUCH A METHOD INCLUDES FIRST PREPARING RELATIVELY LARGE PARTICLES OF THE ALLOY IN THE SIZE RANGE OF ABOUT -20 TO ABOUT -400 MESH. THE PARTICLES ARE THEN TREATED IN AN ATMOSPHERE WHICH PROVIDES ON THE PARTICLE A FILM OF FROM A SMALL BUT EFFECTIVE AMOUNT UP TO ABOUT 6 VOLUME PERCENT OF THE PARTICLE OF A COMPOUND OF THE ALLOY SELECTED FROM THE COMPOUNDS, NITRIDES, CARBIDES AND OXIDES. THE PARTICLES SO TREATED ARE THEN CONSOLIDATED INTO AN ARTICLE, WHICH CAN BE A MILL FORM, DURING WHICH THE FILM IS FRAGMENTED AND THE FRAGMENTS ARE DISPERSED THROUGHOUT THE MATRIX OF THE ARTICLE. FURTHER IMPROVED PROPERTIES ARE OBTAINED BY ADDITIONAL WORKING SUCH AS BY ORDINARY MEANS AS ROLLING, FORMING, SWAGING, ETC. TO PROVIDE A DEFORMATION TEXTURE WHILE PREFERABLY AVOIDING RECRYSTALLIZATION DURING WORKING.

United States Ettent O 3,720,551 METHOD FOR MAKING A DISPERSION STRENGTHENED ALLOY ARTICLE Robert E. Allen, Cincinnati, Ohio, assignor to General Electric Company No Drawing. Filed Jan. 29, 1970, Ser. No. 6,931 Int. Cl. B221 1/00 U.S. Cl. 148-126 4 Claims ABSTRACT OF THE DISCLOSURE A superalloy article based on at least one of the elements from the group iron, cobalt and nickel is provided with improved mechanical strength properties, particularly at about 1400" F. or above, through an improved powder metallurgical method. Such a method includes first preparing relatively large particles of the alloy in the size range of about to about --400 mesh. The particles are then treated in an atmosphere which provides on the particle a film of from a small but elfective amount up to about 6 volume percent of the particle of a compound of the alloy selected from the compounds, nitrides, carbides and oxides. The particles so treated are then consolidated into an article, which can be a mill form, during which the film is fragmented and the fragments are dispersed throughout the matrix of the article. Further improved properties are obtained by additional working such as by ordinary means as rolling, forming, swaging, etc. to provide a deformation texture while preferably avoiding recrystallization during working.

Gas turbine engine designers, during improvement of their product, seek higher temperature capabilities throughout their engines. There are many refractory metal alloys which have structural capabilities at elevated temperatures, for example above 2000 F. However, in gas turbine engines designed to operate in air, the poor surface stability of such refractory metal alloys has prevented their use. Therefore, designers continue to rely on superalloys based on the elements from the group iron, cobalt and nickel.

In particularly wide use are the nickel base superalloys because they can be made to have good surface stability and high strength to temperatures up to about 80% of their absolute melting temperature. Iron and cobalt alloys continue to be used because of their excellent surface stability or hot corrosion resistance, although they are somewhat weaker than the nickel base superalloys. In any event, to meet various advancing design requirements, met-allurgists not only are seeking to find new compositions for such superalloys to provide them with improved mechanical properties along with surface stability but also they are seeking to improve the mechanical properties of known alloys by thermomechanical processing techniques.

It is a principal object of this invention to provide an improved powder metallurgical dispersion strengthening method for enhancing the mechanical strength properties for these superalloys particularly at temperatures of about 1400 F. and above.

Another object is to provide such a method for making a gamma prime strengthened nickel base superalloy article, the mechanical strength properties of which are enhanced through improved dispersion strengthening.

These and other objects and advantages will be more clearly understood from the following detailed description and examples which are typical of rather than limitations on the present invention.

Work reported during the last decade on the powder metallurgy dispersion strengthening at various metals has involved primarily the dispersion of very small size materials added to a powder mixture in various ways. Such materials are or become dispersed refractory compounds, such as oxides, during subsequent processing. The emphasis has been on maintaining the metal particle and the added refractory compound particle very fine with very small interparticle spacing, for example less than about 1 micron.

It has now been discovered, according to the present invention, that major high temperature strengthening elfects can be accomplished without ultra fine dispersons of refractory compounds, for example, oxides, carbides, nitrides or their combinations. Increases in certain high temperature strength properties of at least about 3 times the normal cast or wrought value can be achieved in alloys produced from prealloyed powders in the size range of about 0.001 up to about 0.03" average diameter corre sponding generally to about -400 mesh up to about 20 mesh. Such improvements have been accomplished through powder metallurgy processing of alloys to include relatively coarse, widely separated refractory compounds produced from the alloy particle itself. A result was stress rupture behavior that was as much as 10 parameters on the Larsen-Miller scale better than the ordinary cast or wrought forms of such alloys. In one form of the invention, such improvement is attained through the combination of the use of relatively large particle sizes in combination with careful control over recrystallization behavior during processing to maintain sharp textures and strong deformation structures either with fine elongated grains or large interlocking grains.

The present invention includes first treating the surface of an alloy particle, in the size range of about 0.0015- 0.03 and of the alloy to be strengthened, to provide a film, of a compound of elements of the alloy and selected from the compounds, oxides, nitrides, carbides and their combinations. For example, particles can be oxidized, nitrided or carburized by methods currently used and known in the art. The amount of such film produced is an effective amount of about 0.1-6 volume percent of the particle. Above about 6 volume percent, the resulting alloy is too brittle whereas below about 0.1 volume percent microstructural control is more difficult and insufiicient strengthening is achieved.

After treatment of the alloy particle to provide the surface described above, the particles are consolidated into an article, which as used herein can be a mill stock form or shape, in a manner well known in the art. In one example, they were placed in a steel container which then was provided with a vacuum. The container was heated and then worked by extrusion to consolidate the particles into a bar. One type of container which has been used successfully is about 5" in diameter and about 18" long of mild steel having a wall thickness of about 0.1".

EXAMPLE 1 One alloy used in the evaluation of the present invention was an iron base alloy consisting norminally of, by weight, 25% Cr, 4% Al, 1% Y with the balance essentially iron and incidental impurities. Such an alloy has good ductility and excellent oxidation resistance to temperatures as high as 2600 F. However, it has relatively low tensile and creep properties at temperatures above about 50% of its absolute melting temperature. Therefore, it was considered an excellent example with which to show the improvement obtainable through the present invention without degradation of the excellent surface stability of this alloy system.

Vacuum melted ingots of such iron base alloy were argon atomized and screened under argon to 60 mesh powder. The atomized powder was provided with a thin skin of oxide by heating in air in the range of from 1100- 1600 F. The resulting oxide was predominantly A1 but also included small amounts of such oxides as Fe, A1 0 and Cr O The volume fraction of oxide skin on the powder particle in this example was about 1 volume percent.

The preoxidized powder particles were placed in a mild steel extrusion jacket which was sealed under vacuum. Extrusion was conducted at about 1800 F. to produce a final billet 1.6" by 0.5" by 8'.

Additional preoxidized powders were vacuum hot pressed at 2000 F. under 10,000 p.s.i. for 15 minutes to provide samples for oxidation comparison studies between the oxidized powder and the powder in the as-received condition. Oxidation tests conducted in static air at 2000 F. for 100 hours (cycled to a room temperature every 20 hours) showed very little change in the oxidation characteristics, and hence surface stability, from that of the original alloy. As shown by these data presented in Table I, any difference is predominantly in favor of the greater stability of the oxidized powder.

TABLE I.OXIDATION COMPARISON [Weight gain (mg./crn.

Similar oxidation testing of the extruded material yielded substantially identical data.

The surface stability characteristics of the alloy powder was substantially unaffected by its being oxidized prior to compaction and extrusion. However, its mechanical pronerties were greatly improved as a result of the combination of such preliminary oxidation and subsequent thermomechanical processing. The data of Table II compares the ordinary cast and wrought form of the alloy listed as Condition A with the alloy resulting from the present invention listed as Condition B and Condition C. The condition of Example B is the alloy in the as-extruded form previously described. The condition of Example C is the alloy after extrusion (primary working) and rolling (secondary working) at 1800 F. with multiple passes and intermediate soaks at 1200" F. until a total reduction from secondary working of at least 75% had been attained. The tensile specimens were heat treated in air at 2200 F. for 2 hours prior to testing.

TABLE II [2,000 F. testing] Tensile Stress rupture Alloy Ultimate 0.2 yield Elong. Stress Life condition (p.s.i.) (p.s.i.) (percent) (p.s.i.) (hrs) As was mentioned before, the present invention very significantly improves the mechanical strength characteristics of a resulting alloy article. It is easily seen from Table II by comparison between the ordinary wrought or cast form of the alloy, represented by Condition A, with other forms of the alloy resulting from the practice of the present invention. In its preferred form in which sharp textures and strong deformation structures are provided as a result of working, either with fine elongated grains or large interlocking grains, significant additional improvement in the mechanical properties can be attained. This is shown by a comparison between alloy Conditions B and C in Table II. Condition C provides such a structure as a result of rolling up to a total of about 88% reduction whereas Condition B represented the as-extruded form.

4 EXAMPLE 2 TABLE III [1,800 F. tensile testing] Volume percent 1,800 F. data Alloy Ultimate 0.2% yield Elong. condition Oxide Carbide (K .s.i.) (K.s.i.) (percent) D 0 1.0 55 35 E 2.0 1. 0 149 137 7 F 0 1. 5 142 16 NOTE: K.s.i. means 1,000 lbs/infi.

The ordinary condition and tensile properties for this alloy in its wrought condition is shown as Condition D. The alloy was worked at about 2050 F. to reduce it, without control of recrystallization. Therefore, the alloy was generally recrystallized between working steps.

The alloy conditions resulting from practice of the present invention are shown by wrought forms E and F. These forms were extruded in steps to a 128/1 ratio in the range of 1900-2000 F., taking care during steps not to recrystallize. Then, after working, these forms were recrystallized at about 2200 F. Thus, aside from such care during working, the heat treatments for all conditions of Table III were the same.

As can be seen from the data of Table III, this Example 2 alloy is greatl improved by the combination of oxides or carbides or both, included through pretreatment, with thermomechanical processing which avoids recrystallization during working. Both the ultimate and 0.2% yield strengths are improved at least about 3 times.

The alloy of this example, currently used in production parts for gas turbine engines, is typical of the gamma prime strengthened, nickel base superalloys which are sensitive to recrystallization during working. Thus, a preferred form of this invention, particularly as it relates to such Ni-base superalloys, includes the combination of compound addition and control of recrystallization during working.

What is claimed is:

1. In a method for making an article from an alloy useful at least at about 1400 F. and based on at least one element selected from the group consisting of Fe, Co, and Ni, the steps of:

(a) preparing from the alloy a plurality of particles in the size range of about 0001-003" average diameter;

(b) heating the particles in an atmosphere for a time sufiicient to provide on surfaces of the particles, from elements of the alloy, a film of a refractory compound selected from the group consisting of carbides, nitrides, oxides and their combinations, the film being from a small but effective amount of about 0.1 up to about 6 volume percent of the particles;

(c) hot consolidating the particles into an article during which substantially no recrystallization occurs, while fragmenting the film and dispersing fragments of the film through the article; and then ((1) recrystallizing the alloy.

2. The method of claim 1 in which the heating of the particles is conducted in the range of about 1100-1600 F. and consolidation is conducted in the range of about 1800-2000" F.

3. The method of claim 1 in which the alloy is secondarily worked, after consolidating and before recrystallization, in an amount equal to that attained by a total reduction of at least about 75%.

4. The method of claim 3 for making a gamma prime strengthened nickel base alloy article which:

heating of the particles is conducted in the range of about 1100-1600 F.; the film is in an amount of about 1-3 volume percent; the consolidation is conducted in the range of about 1800-2000 F.; the article is secondarily worked in the range of about 1900-2000 F.; and recrystallization is conducted in the range of about References Cited UNITED STATES PATENTS 6 3,026,200 3/ 1962 Gregory 75206 3,315,342 4/ 1967 Roberts 75206 3,189,989 6/1965 Ebdon 75-206 3,343,952 9/1967 Delgrosso et al 75212 3,322,536 5/1967 Stoddard et a1 75212 3,216,824 11/1965 Boghen et al 75212 3,073,698 1/ 1963 Arbiter 75--212 FOREIGN PATENTS 866,082 4/ 1961 Great Britain 75206 CARL D. QUARFORTH, Primary Examiner B. HUNT, Assistant Examiner U.S. C1. X.R. 

